1. Field of the Invention
This invention relates to a single-sided circuit substrate for a multilayer printed wiring board, a multilayer printed wiring board and a method of manufacturing the same, and more particularly, it proposes a single-sided circuit board to be developed for manufacturing a multilayer printed wiring board having an interstitial-viahole structure in higher yield and efficiency, and a multilayer printed wiring board produced by laminating a plurality of such single-sided circuit boards, and a method of manufacturing the same.
2. Description of Background Information
The conventional multilayer printed wiring board is a multilayered body constituted by alternately building up copper-clad laminates and prepregs. This multilayered body has a surface wiring pattern on an outer surface thereof, and has an innerlayer wiring pattern between its interlaminar layers. These wiring patterns are electrically connected via through-holes formed in the multilayered body in a thickness direction thereof between the mutual innerlayer wiring patterns, or between the innerlayer wiring pattern and the surface wiring pattern.
However, in the multilayer printed wiring board having the above-mentioned through-hole structure, it is required to ensure a region for the formation of the through-holes, so that it is difficult to attain a high densification of mounting parts and hence there are disadvantages that it is not able to fully meet the requirements of super-miniaturization of portable electric equipments and practical application of the narrow-pitched packages and MCM (Multi Chip Module).
Recently, instead of the multilayer printed wiring board with the through-hole structure as mentioned above, a multilayer printed wiring board having an interstitial-viahole (IVH) structure has been noticed, which is suitable for the miniaturization and high densification of the electric equipment.
The multilayer printed wiring board having the IVH structure is a printed wiring board having such a structure that conductive viaholes connecting conductor layers to each other are arranged in each of the interlaminar insulating layers forming the multilayered body. That is, in such wiring board, electrical connection between the mutual innerlayer wiring patterns or between the innerlayer wiring pattern and the surface wiring pattern is made through viaholes (buried viaholes or blind viaholes) not passing the wiring substrate. Therefore, in the multilayer printed wiring board having the IVH structure, it is not required to provide specific region for the formation of the through-holes, and hence the miniaturization and high densification of the electric equipment can easily be attained.
As to the multilayer printed wiring board having the IVH structure, there is reported a proposal for the development of the multilayer printed wiring board having the IVH structure for all layers, for instance, on page 57 of "The 9th National Convention Record JIPC (Mar. 2, 1995)" or a draft collection for 9th Circuit Packaging Academic Lecture. The multilayer printed wiring board according to this proposal is developed on the basis of 1 high-speed drilling technique of minute viaholes by carbon dioxide gas laser, 2 adoption of a composite material of aramide nonwoven fabric and epoxy resin as a substrate material, and 3 interlaminar connecting technique by filling of a conductive paste, and manufactured by the following processes.
At first, an aramide nonwoven fabric impregnated with epoxy resin is used as a prepreg and hole drilling through carbon dioxide gas laser is applied to the prepreg, and then a conductive paste is filled in the resulting hole portions (see FIG. 1(a)).
Next, copper foils are put on both surfaces of of the prepreg, and pressed under heating through heat press. Thus, the epoxy resin in the prepreg and the conductive paste are hardened to provide an electrical connection between the copper foils put on both surfaces of the prepreg (see FIG. 1(b)).
And then, patterning is carried out onto the copper foils by an etching process to provide double-sided hard substrate having viaholes (see FIG. 1(c)).
The formation of multilayers is carried out by using the double-sided substrate as a core layer. Concretely, the prepreg filled with the above conductive paste and the copper foil are successively laminated on both surfaces of the core layer while positioning and again heat-pressed and then the copper foil of the outermost layer is etched to provide a four-layer substrate (see FIGS. 1(d), (e)). If it is intended to further conduct the formation of the multilayer, the above steps are repeated to obtain a six or eight-layer substrate.
The drawback of the above conventional technique is that the production steps become complicated and a long time is taken for the production because the heating and pressing through hot press and the patterning step of the copper foil through etching should be repeated at any number of times.
In the multilayer printed wiring board having the IVH structure obtained by the above production method, it is difficult to confirm the poor patterning of the copper foil in the production course, so that if the poor patterning is caused even at one place (one step) in the production course, the whole of the wiring board as a final product becomes inferior.
That is, the above conventional production process has a fatal drawback that the degradation of production efficiency or production yield is liable to be caused because if inferior good is produced even at one place among the lamination steps, good products made in the other lamination steps should be removed.
It is an object of the invention to provide a single-sided circuit substrate for multilayer printed wiring board developed for efficiently manufacturing a multilayer printed wiring board having an IVH structure in a high yield.
It is another object of the invention to provide a multilayer printed wiring board having an IVH structure constituted with the above single-sided circuit substrates.
It is the other object of the invention to propose a method of efficiently manufacturing the multilayer printed wiring board having the IVH structure in a higher yield by using the single-sided circuit substrates.